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Al laminate by a porthole die co-extrusion process
Accepted Manuscript Title: Fabrication of Al/Mg/Al laminate by a porthole die co-extrusion process Authors: Liang Chen, Jianwei Tang, Guoqun Zhao, Cunsheng Zhang, Xingrong Chu PII: DOI: Reference:
S0924-0136(18)30132-8 https://doi.org/10.1016/j.jmatprotec.2018.03.027 PROTEC 15697
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
29-1-2018 16-3-2018 28-3-2018
Please cite this article as: Chen L, Tang J, Zhao G, Zhang C, Chu X, Fabrication of Al/Mg/Al laminate by a porthole die co-extrusion process, Journal of Materials Processing Tech. (2010), https://doi.org/10.1016/j.jmatprotec.2018.03.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of Al/Mg/Al laminate by a porthole die co-extrusion process Liang Chena, Jianwei Tanga, Guoqun Zhaoa,*, Cunsheng Zhanga, Xingrong Chua a
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry
* Corresponding author: Guoqun Zhao, Professor @ Shandong University.
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Email: [email protected], Tel: +8653181696577, Fax: +8653188392811.
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of Education), Shandong University, Jinan, Shandong 250061, PR China.
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Abstract
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Graphical Abstract
A porthole die co-extrusion (PCE) process was proposed to fabricate the Al/Mg/Al
laminate. The results showed that the laminate was successfully extruded without voids and
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cracks on the Al/Mg interface. The transition layer was formed and its thickness was increased with the increase of temperature. Partial dynamic recrystallization (DRX) occurred in Al layer, and the texture of Al layer has strong E {111}<011> and Y {111}<112> sheartyped components and relative weak Copper {112}<111> and S {123}<634> rolling components. The near complete DRXed grain structure was observed in Mg layer, and the average grain size increases with increasing temperature. Mg layer has strong basal plane
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texture with (0001) planes parallel to extrusion direction and prismatic planes with random distribution. At lower extrusion temperature, both the Al and Mg matrixes exhibited higher hardness. Furthermore, the hardness of Al/Mg interface is lower than that of the Al and Mg matrix. Keywords: Porthole die co-extrusion; Al/Mg/Al laminate; Microstructure; Mechanical
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property.
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1. Introduction
The consumption of Mg alloy is increasing in the fields of automobile and aerospace due to the advantages of low density, high mechanical properties, excellent electromagnetic shielding ability and good recyclability (Meng et al., 2017). However, the practical application of Mg alloy is limited by the concern about its poor corrosion resistance. Similar to Mg alloy, Al alloy is also a light weight material with high strength and excellent corrosion
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resistance (Paulisch et al., 2016). According to the previous study of Li et al. (2016),
Al/Mg/Al laminate is regarded as an effective method to improve the corrosion resistance and
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the formability of Mg alloy. With the demand of light weight design, the Al/Mg/Al laminate has important applications in the fields of aerospace, automobile and high speed train by replacing the utilization of Al alloy.
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The researchers have devoted their efforts to finding effective method for joining
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dissimilar Al/Mg alloys. Liu et al. (2009) reported that Al/Mg alloys can be joined through
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diffusion bonding method, and the formation of intermetallic phases was related to holding
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time. Yan et al. (2010) fabricated a composite Al/Mg plate by means of explosive welding. Liang et al. (2017) applied friction stir welding (FSW) to join 1060 Al and AZ31 Mg alloys,
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and found that the constitutional liquation occurred at the interface of Al/Mg joints. Zhang et al. (2010) fabricated an Al/Mg/Al composite with a trilaminate structure by hot rolling, and
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investigated its mechanical properties at quasi-static rates of strain. Chang et al. (2009) reported that Al/Mg laminate can be fabricated by accumulative roll bonding, and studied the
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texture evolution of both Al and Mg layers. Similarly, Wu et al. (2010) studied the microstructure and mechanical properties of Al/Mg laminate fabricated by accumulative roll bonding.
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Recently, the researchers attempted to fabricate Al/Mg composite through hot extrusion
process. Both Mg and Al alloys are favorable for hot extrusion, and the Al/Mg composite can be directly extruded from as-cast Mg and Al billets. Thus, it is expected that hot extrusion can be used in mass-production of Al/Mg composite due to its high efficiency. Xin et al. (2015) developed an accumulative extrusion bonding (AEB) process to produce Al/Mg multilayer plates with excellent layer-layer bonding, and studied the grain structure and textures of both
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layers. Tokunaga et al. (2014) placed an Al plate on the top of Mg billet, and successfully obtained a bar with Mg core and uniform Al coating. Mahmoodkhani et al. (2016) studied the material flow and metallurgical reactions during co-extrusion of an Al/Mg clad. On the other hand, the researchers have proved that the hybrid billet combining Al sleeve and Mg core is effective in producing Al/Mg composite. Feng et al. (2016) extruded a well bonded Al/Mg rod using the as-cast billet including 7050 Al sleeve and AZ31 Mg core. Negendank et al.
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(2012) investigated the effects of die angle on the formation of diffusion layer during
extrusion using hybrid Al/Mg billet. Priel et al. (2016) designed the configuration of the
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hybrid billet, and the Al/Mg composite rod with no cracks, tears, and voids was obtained. The above mentioned studies proved that the Al/Mg composite with good bonding interface can be successfully hot extruded. However, it is noticed that the preparation of hybrid billet
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especially the Al sleeve is difficult and wasteful. Besides, the impurity and oxidation between
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Al and Mg billets might have adverse effects on the bonding quality of the Al/Mg interface.
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In the authors’ recent study (Fan et al., 2017), the porthole die extrusion was employed
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to join dissimilar 1060/6063 Al alloys, and a sound bonding interface was obtained. During such process, the surfaces of 1060 and 6063 Al billets were remained in the container, and
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only the fresh metal streams were solid bonded in the welding chamber under the condition of high temperature and pressure. Thus, the influence of oxidation on bonding quality was
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avoided. Moreover, the billets have simple cylindrical shape, which is easy for preparation. In order to further improve the boding quality, the structure of porthole die and process
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parameters such as ram velocity, extrusion ratio, and billet temperature should be well designed and controlled (Chen et al., 2015; Zhang et al., 2017a). In this study, a porthole die co-extrusion (PCE) process was proposed to fabricate the
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Al/Mg/Al laminate. The extrusion experiments were carried out at varied temperatures of 340 o
C, 370 oC and 400 oC, respectively. The morphology of Al/Mg interface and element
diffusion were analyzed. The grain structure and micro-texture in Al layer and Mg layer adjacent the interface were well examined. The micro-hardness across the bonding interface was measured. Based on the above works, the effects of billet temperature on microstructure and hardness of the extruded laminate were discussed. Importantly, it is proved that the
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Al/Mg/Al laminate with good bonding interface can be obtained through PCE method, and the microstructure of both Al and Mg layers was improved. 2. Experimental procedures
The 6063 Al and AZ91 Mg were used to fabricate Al/Mg/Al laminate, and the chemical compositions of both alloys are listed in Table 1. The as-cast 6063 ingot was homogenized at
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480 oC for 12 h and then cooled down to room temperature in the air. Similarly, the homogenization for as-cast AZ91 ingot was carried out at 420 oC for 10 h followed by air
cooling. Then, the cylindrical billets for 6063 and AZ91 alloys were machined for extrusion
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experiment.
Table 1 Chemical compositions (wt.%) of the as-received AZ91 Mg alloy and 6063 Al alloy. Si
Fe
Cu
Mn
Zn
Al
Mg
AZ91 Mg
0.031
-
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0.33
0.64
9.31
Bal.
6063 Al
0.45
0.35
0.10
0.12
0.12
Bal.
0.80
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Alloy
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The designed experimental setup and the material flow behavior during the proposed PCE process are shown in Fig. 1, where ED and TD indicate the extrusion direction and
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transverse direction, respectively. As is seen, the setup mainly consists of the components of ram, container, upper die and lower die. Three cavities with the inner diameter of 20 mm were
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designed in the container. Accordingly, the upper die has three portholes with the inner diameter of 14 mm and the welding chamber with the height of 10 mm. The bearing in lower
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die is used to control the final dimension of the extruded laminate. In this study, the plateshaped Al/Mg/Al laminate with the rectangular cross-section of 18×5 mm is extruded out
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from die bearing.
The PCE experiments were conducted using an electro-hydraulic servo pressing machine
of 200 tons. Firstly, two Al billets (ϕ20×62 mm) and one Mg billet (ϕ20×52 mm) were placed into the cavities of the container. The height of Mg billet is lower than that of the Al billet, in order to ensure that all metal streams can be extruded out simultaneously. Then, the billets, container, and dies were heated to the experimental temperature by a heating coil and held for
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15 min to have a uniform temperature distribution. After that, with the pressure from the stem, the fresh metal streams flowed into the portholes, and were solid bonded inside the welding chamber. Finally, the Al/Mg/Al laminate was extruded out from the die bearing. After extrusion experiment, the dies and extruded laminate were immediately quenched into
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iced water.
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Fig. 1. The experimental setup and material flow behavior during PCE process
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The process parameters such as ram velocity, extrusion ratio, billet temperature, and
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container temperature significantly affect the microstructure and the bonding quality of the extruded profile. In this study, the effect of billet temperature was studied, and the PCE experiments were performed at various temperatures of 340 oC, 370 oC and 400 oC.
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Accordingly, the extruded laminates were designated as PCE340, PCE370, and PCE400,
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respectively. The morphology and thickness of Al/Mg bonding interface were examined using scanning electron microscopy (SEM), and the element distribution across the Al/Mg interface was analyzed by means of energy dispersive spectroscopy (EDS). The grain structure and micro-texture of the Al and Mg layers near the Al/Mg interface were examined using electron
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back-scattered diffraction (EBSD). The Al and Mg specimens for EBSD analysis were firstly grounded and mechanically polished. Then, the Al specimens were electro-polished in the solution of 10 ml perchloric acid and 90 ml ethanol at 20 V for 5 s, and the Mg specimens were electro-polished using the mixture of 800 ml ethanol, 100 ml propanol, 18.5 ml distilled water, 10 g hydroxyquinoline and 75 g citric acid (ACII electrolyte) with the cooling of liquid nitrogen at 20 V for 40 s. The EBSD data was analyzed by Channel 5 software. Moreover, the 6
micro-hardness of the extruded laminate was measured at the interval of 0.5 mm along TD direction. 3 Results and discussion 3.1 Al/Mg bonding interface
The morphology of Al/Mg bonding interface under various experimental temperatures is
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shown in Fig. 2, where the green and red points represent the distribution of Al and Mg elements, respectively. As is seen, an obvious transition layer without any voids and cracks is formed in the interface of all laminates, which implies that a sound interfacial bonding was
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obtained by the proposed PCE process. Moreover, the transition layer contains two distinct sub-layers, and the sub-layer close to Al layer is thicker than that close to Mg layer. This
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phenomenon is similar with the experimental results reported by Wu et al. (2015).
Fig. 2. SEM micrographs and EDS mappings across Al/Mg interface of (a) PCE340, (b)
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PCE370 and (c) PCE400. Twelve positions along ED direction of the extruded laminate were selected to measure
the thickness of transition layer, and the results is shown in Fig. 3. It is obvious that the thickness of transition layer is increased with increasing experimental temperature. The average thicknesses of transition layers are calculated as 1.26 m, 2.26 m, 2.97 m for laminates PCE340, PCE370 and PCE400, respectively. Based on the element distribution 7
shown in Fig. 2, the inter-diffusion of Al and Mg atoms occurred during PCE process. The diffusion distance was increased with increasing temperature, which results in the variation of transition thickness with temperature. It is known that the diffusion in solid materials can be described by Fick’s second law, and the diffusion coefficient becomes higher with increasing temperature. On the other hand, the transport of atoms usually occurs though flaws and gaps in the solid materials. During PCE process, large amount of dislocations and vacancies were
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formed in the bonding interface due to the severe plastic deformation. These additional
vacancies have low migration energy, since high pressure was applied on the material. Hence,
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the vacancies tended to move towards the defects such as dislocations and grain boundaries, and the system energy can be reduced by such kind of vacancy eliminating (Sauvage et al., 2005; Ma et al., 2015). The equilibrium concentration of vacancies is enhanced at higher
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temperature, according to the following equation,
Ev kT
(1)
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Cv Ae
where Cv is the equilibrium concentration of vacancies, A is a coefficient determined by
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vibrational entropy, k is boltzmann constant, T is temperature, and Ev is vacancy formation
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energy. As a result, with the increase of temperature, more vacancies are induced, which can accelerate the atom diffusion across the Al/Mg interface. On the other hand, the diffusion velocity of Mg atoms into Al matrix is faster than that of Al atoms into Mg matrix as reported
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by Negendank et al. (2012), which results in the thickness of sub-layer close to Al matrix is
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larger than that close to Mg matrix. Fig. 4 plots the EDS line analysis across the Al/Mg interface. In the transition layer, Al concentration decreases from Al matrix to Mg matrix, while the tendency of Mg concentration is opposite. The concentrations of Al and Mg vary continuously across the Al/Mg interface. According to the previous study of Matsumoto et al.
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(2005), it implies that the materials were miscible at the present temperature and the intermetallic was not formed in the transition layer. The similar results were reported by Zhang et al. (2011) that the intermetallic was not formed in the interface of 7075 Al and AZ31B Mg alloys fabricated by hot rolling.
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Fig. 3. Thickness of Al/Mg interface of (a) PCE340, (b) PCE370 and (c) PCE400.
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Fig. 4. EDS line scanning across Al/Mg interface of (a) PCE340, (b) PCE370, and (c) PCE400.
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3.2 Microstructure in Al layer
The microstructure in Al layer adjacent Al/Mg interface was examined by means of
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EBSD. The inverse pole figure maps of Al layer are shown in Fig. 5, where the high angle boundaries (HABs) with misorientation angles >15° are indicated by thick-black lines and low angle boundaries (LABs) with misorientation angles between 2°-15° are indicated by
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thin-grey lines, respectively. Moreover, the fraction of HABs is defined by f. As is seen, Al layer of each laminate exhibits a bimodal grain structure consisting of coarse elongated grains and fine equiaxed grains, which implies the occurrence of partial dynamic recrystallization (DRX). Moreover, large amounts of LABs were formed inside the elongated grains, which is the evidence of continuous dynamic recrystallization (CDRX) (Jamaati and Toroghinejad, 2014). The evolution of grain structure in Al layer during PCE process is explained as below.
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Firstly, the coarse equiaxed grains of the billet were elongated along ED direction because of the compression stress, and those elongated grains were transformed to strip-shaped coarse grains. Then, a large amount of LABs was formed due to the occurrence of dynamic recovery. Finally, the dislocations produced by strain hardening accumulated in the LABs, which leads to the transformation from LABs to HABs and the occurrence of CDRX. As a result, the extruded microstructure of Al layer consists of strip-shaped coarse grains and some amount of
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fine equiaxed grains.
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Fig. 5. Inverse pole figure maps of the grain morphology in Al layer of (a) PCE340, (b) PCE370, and (c) PCE400.
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Fig. 6 shows the grain size distribution and the volume fraction of DRX in Al layer. As
is seen, the average grain size increases from 4.05 m to 6.72 m with increasing temperature. The smallest grain size in PCE340 laminate is firstly attributed to the finer
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DRXed grains and low mobility of grain growth at lower temperature. Moreover, the severe pressure and shearing action occurred during bonding stage, which induces large amount of dislocations to the grains near the bonding interface. The elongated grains can be broken by means of dislocation cutting and grain rotation (Cheng et al., 2014). In this study, the pressure and shearing action becomes much stronger at lower extrusion temperature, which is favorable for the formation of fine grains by breaking the elongated grains. Thus, due to the
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finer DRXed grains, low grain growth rate and breaking of elongated grains, PCE340 laminate has the smallest grain size. Fig. 6 (d) shows that the volume fraction of DRX rises with increasing temperature. It is known that high deformation temperature is beneficial for the DRX occurrence of Al alloys (Chen et al., 2017). During higher temperature deformation, more energy is provided for the transformation of microstructure, which can promote the growth of sub-grains and the nucleation of new grains. Besides, it will be easier for
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dislocation to slip and climb at higher temperature, and such dislocation migration can lead to
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the merging of sub-grains and the transformation of LABs to HABs (Liao et al., 2017).
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Fig. 6. Grain size distributions in Al layer of (a) PCE340, (b) PCE370 and (c) PCE400, and the (d) volume fraction of DRX. The micro-texture in Al layer is analyzed based on the orientation distribution function
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(ODF) sections of φ2=0°, 45° and 65°, as shown in Fig. 7. The locations of main ideal orientations in ODF section are given in Fig. 8. The strong E {111}<011> and Y {111}<112> components and relative weak Copper {112}<111> and S {123}<634> components can be observed in all extruded laminates. Besides, PCE340 and PCE400 have weak R-Cube component, while PCE370 has weak R-Goss component. The fraction of main texture components in Al layer is listed Table. 2. The sum of fraction of shear-typed components
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such as E and Y is 57.7%, 72.2% and 68.05% for PCE340, PCE370 and PCE400, respectively. The sum of fraction of rolling texture such as Copper and S is 14.18%, 9.12% and 13.23% for PCE340, PCE370 and PCE400, respectively. Consequently, the main textures of Al layer are characterized as shear-typed components, and also small fraction of rolling texture. The dominance of deformation textures in Al layer also indicates the occurrence of CDRX, which agrees well with the above discussion. In this study, strong shearing stress is
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imposed on Al alloy due to the friction and die constraint before bonding stage, and strong shear-typed textures are formed by the rotation of grains. These shear-typed E and Y
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components can transform into each other under combined effects of shearing stress, shear strain rate and temperature. However, during the bonding stage, some of the shear-typed textures transform to rolling textures due to the action of plane strain. Moreover, this
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phenomenon can contribute to an increase in the fraction of HABs (Kim et al., 2005). For
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PCE340, more shear-typed textures transform into rolling textures, which leads to a higher
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fraction of HABs, as shown in Fig. 9. The maximum texture intensities of PCE340, PCE370 and PCE400 are 4.67, 8.01 and 7.76, respectively. This is because the transformation degree
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from shear-typed to rolling texture. More shear-typed textures transform to rolling textures for
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PCE340, which lead to the lowest value of texture intensity.
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Fig. 7. The φ2=0°, 45° and 65° ODF sections of (a) PCE340, (b) PCE370 and (c) PCE400.
Fig. 8. Schematic illustration of the main texture components in ideal FCC materials.
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Table 2 The fractions of texture components in the Al layer.
Specimen
Texture component (%) E
Y
Copper
S
{111}<011>
{111}<112>
{112}<111>
{123}<634>
PCE340
40.10
17.60
6.94
7.24
PCE370
56.80
15.40
3.28
5.84
PCE400
59.60
8.45
7.73
5.50
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Fig. 9. Relative frequency of misorientation angles in Al layer of (a) PCE340, (b) PCE370
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and (c) PCE400.
3.3 Microstructure in Mg layer
Fig. 10 shows the grain morphology in Mg layer near the Al/Mg interface. The Mg layer
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of all laminates exhibits uniform and fine equiaxed grains with the size of several microns,
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which can be attributed to occurrence of complete DRX. The average grain sizes of PCE340,
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PCE370 and PCE400 are calculated as 2.59 m, 3.24 m and 3.66 m, respectively. Fig. 11
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shows the distribution of second phases in Mg layer. It can be observed that both coarse and relative fine Mg17Al12 particles distribute along the grain boundaries. Moreover, the amount
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of fine Mg17Al12 particles in PCE340 is higher than those of PCE370 and PCE400. In case of AZ series Mg alloy, Al atoms dissolved into Mg matrix during the homogenization treatment
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resulting in the Al segregation, and the Mg17Al12 precipitations can be dynamically formed in the Al enriched region during the following hot extrusion process (Kim et al., 2017). With the
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increase of temperature, the solid solubility of Al atoms in Mg matrix is enhanced, and the amount of Mg17Al12 precipitations is significantly reduced. The grain size of Mg alloys after extrusion is strongly dependent on the amount and distribution of second phase precipitations.
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The Mg17Al12 precipitates can promote the DRX nucleation, which is well known as the particle stimulated nucleation (PSN) effects. Moreover, the Mg17Al12 precipitates strongly inhibit the growth of DRXed grains by reducing the interfacial energy of grain boundaries. On the other hand, the grain growth might also occur after DRX, while the driving force for grain growth is reduced at low temperature. Due to the above mentioned reasons, the grain size in Mg layer is increased with increasing temperature.
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PCE370, and (c) PCE400.
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Fig. 10. Inverse pole figure maps of the grain morphology in Mg layer of (a) PCE340, (b)
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Fig. 11. SEM micrographs of the precipitate distribution in Mg layer of (a) PCE340, (b) PCE370, (c) PCE400. The (0001) and (10-10) pole figures of Mg layer are plotted in Fig. 12. Extrusion temperature has significant effects on the texture evolution of Mg alloys. As is seen, all laminates have a strong basal plane texture with (0001) planes parallel to ED direction, which
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is a typical texture for extruded Mg alloy as reported by Stanford and Barnett (2008). Besides, the prismatic planes with random distribution can also be observed. The basal pole has the
tendency to rotate away from TD direction. It is attributed to the activity of non-basal
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slip (Agnew et al., 2001), which is beneficial for the improvement of forming ability. The maximum texture intensity of PCE340, PCE370 and PCE400 are 4.94, 7.79 and 7.82,
respectively. Although complete DRX occurred in all laminates, the DRXed grains are finest
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in PCE340. Borkar et al. (2013) reported that fine DRXed grains have relatively more random
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orientations than medium to large sized grains. Thus, a weaker texture was formed in PCE340
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extruded at low temperature. With increasing temperature, the number of fine grains with
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random orientation decreases, and the maximum texture intensity is increased.
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Fig. 12. Pole figures in Mg layer of (a) PCE340, (b) PCE370 and (c) PCE400.
3.4 Micro-hardness
Fig. 13 plots the micro-hardness distribution across the Al/Mg interface along TD direction. The values of hardness are almost stable in the zones far away from the Al/Mg
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interface. It is noted that PCE340 extruded at lower temperature exhibits higher hardness in
the matrix of both Al and Mg layers than that of PCE370 and PCE400. As discussed above,
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the average grain size in both Al and Mg layers increases with increasing temperature, which results in the reduction of hardness (Yu et al., 2017). Moreover, the DRX degree also
increases with increasing temperature in both Al and Mg layers. The occurrence of DRX can
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reduce the density of dislocations, and the work hardening during hot extrusion can be well relieved with higher DRX degree, which also result in the reduction of hardness. It is also
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observed that the hardness on Al/Mg interface is lower than that of the matrix zones.
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Generally, the hardness of 6063 Al alloy is in the range of 40-90 HV, and the hardness of
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AZ91 Mg alloy is in the range of 70-170 HV (Zhang et al., 2017b; Roy et al., 2015). It is mentioned that the intermetallics were not formed in the transition layer, and it is not
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expected that the transition layer exhibits high hardness.
Fig. 13. Hardness distributions across the Al/Mg bonding interface of the extruded laminates.
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4. Conclusions
In this study, a PCE method was proposed to fabricate the Al/Mg/Al laminate. The effects of temperature on the Al/Mg interface and microstructure of the extruded laminates were investigated. The main conclusions are drawn as follows. (1) The Al/Mg bonding interface without voids and cracks was obtained using the
formed and its thickness increased with the increase of extrusion temperature.
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proposed PCE process. Due to the diffusion of Al and Mg atoms, the transition layer was
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(2) The microstructure of Al layer consists of both coarse elongated and fine equiaxed
grains, which indicates the occurrence of partial DRX. Fine grains with uniform distribution were observed in Mg layer due to the complete DRX. Moreover, the grain size of both Al and
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Mg layers increased with the increasing temperature.
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(3) The texture in Al layer consists of strong E {111}<011> and Y {111}<112> shear-
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typed components and relative weak Copper {112}<111> and S {123}<634> rolling components. Mg layer has strong basal plane texture with (0001) planes parallel to ED
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direction, and also some prismatic planes with random distribution
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(4) The micro-hardness in both Al and Mg matrixes is higher after low temperature extrusion due to the finer grain size. The hardness in the Al/Mg interface is lower than that in
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the matrix, since the intermetallics were not formed.
Acknowledgements The authors would like to acknowledge the financial support from National Natural Science
Foundation of China (U1708251), China Postdoctoral Science Foundation (2017M622194) and The
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Fundamental Research Funds of Shandong University (2017JC005).
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interface of Al/Mg friction welding joints. Sci. Technol. Weld. Join. 22, 363-372.
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Liao, B., Wu, X., Yan, C., Liu, Z., Ji Y., Cao, L., Huang, G., Liu, Q., 2017.
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Microstructure characterization of Al-cladded Al-Zn-Mg-Cu sheet in different hot
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deformation conditions. Trans. Nonferrous Met. Soc. China 27, 1689-1697. Liu, L., Zhao, L., Xu, R., 2009. Effect of interlayer composition on the microstructure
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and strength of diffusion bonded Mg/Al joint. Mater. Des. 30, 4548-4551. Ma, M., Huo, P., Liu, W., Wang, G., Wang, D., 2015. Microstructure and mechanical
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properties of Al/Ti/Al laminated composites prepared by roll bonding. Mater. Sci. Eng. A
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636, 301-310.
Mahmoodkhani, Y., Wells, M., 2016. Co-extrusion process to produce Al-Mg eutectic
clad magnesium products at elevated temperatures. J. Mater. Process. Technol. 232, 175-183.
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Matsumoto, H., Watanabe, S., Hanada, S., 2005. Fabrication of pure Al/Mg-Li alloy clad
plate and its mechanical properties. J. Mater. Process. Technol. 169, 9-15. Meng, Y., Chen, Q., Sugiyama, S., Yanagimoto, J., 2017. Effects of reheating and subsequent rapid cooling on microstructural evolution and semisolid forming behaviors of extruded Mg-8.20Gd-4.48Y-3.34Zn-0.36Zr alloy. J. Mater. Process. Technol. 247, 192-203.
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Negendank, M., Mueller, S., Reimers, W., 2012. Coextrusion of Mg-Al macro composites. J. Mater. Process. Technol. 212, 1954-1962. Paulisch, M.C., Lentz, M., Wemme, H., Andrich, A., Driehorst, I., Reimers, W., 2016. The different dependencies of the mechanical properties and microstructures on hot extrusion and artificial aging processing in case of the alloys Al 7108 and Al 7175. J. Mater. Process.
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Technol. 233, 68-78. Priel, E., Ungarish, Z., Navi, N.U., 2016. Co-extrusion of a Mg/Al composite billet: A
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computational study validated by experiments. J. Mater. Process. Technol. 236,103-113.
Roy, S., Kannan, G., Suwas, S., Surappa, M.K., 2015. Effect of extrusion ratio on the microstructure, texture and mechanical properties of (Mg/AZ91)m-SiCp composite. Mater.
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Sci. Eng. A 624, 279-290.
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Sauvage, X., Wetscher, F., Pareige, P., 2005. Mechanical alloying of Cu and Fe induced
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by severe plastic deformation of a Cu-Fe composite. Acta Mater. 53, 2127-2135.
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Stanford, N., Barnett, M., 2008. Effect of composition on the texture and deformation
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behaviour of wrought Mg alloys. Scr. Mater. 58, 179-182. Tokunaga, T., Matsuura, K., Ohno, M., 2014. Superplastic behavior of Al-coated Mg
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alloy sheet. J. Alloy. Compd. 601, 179-185. Wu, K., Chang, H., Maawad, E., Gan, W., Brokmeier, H.G., Zheng, M., 2010.
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Microstructure and mechanical properties of the Mg/Al laminated composite fabricated by accumulative roll bonding (ARB). Mater. Sci. Eng. A 527, 3073-3078. Wu, Y., Feng, B., Xin, Y., Hong, R., Yu, H., Liu, Q., 2015. Microstructure and
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mechanical behavior of a Mg AZ31/Al 7050 laminate composite fabricated by extrusion. Mater. Sci. Eng. A 640, 454-459. Xin, Y., Hong, R., Feng, B., Yu, H., Wu, Y., Liu, Q., 2015. Fabrication of Mg/Al multilayer plates using an accumulative extrusion bonding process. Mater. Sci. Eng. A 640, 210-216.
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Yan, Y., Zhang, Z., Shen, W., Wang, J., Zhang, L., Chin, B., 2010. Microstructure and properties of magnesium AZ31B-aluminum 7075 explosively welded composite plate. Mater. Sci. Eng. A 527, 2241-2245. Yu, J., Zhao, G., Zhang, C., Chen, L., 2017. Dynamic evolution of grain structure and micro-texture along a welding path of aluminum alloy profiles extruded by porthole dies.
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Mater. Sci. Eng. A 682, 679-690. Zhang, J., Chen, L., Zhao, G., Zhang, C., Zhou, J., 2017a. Study on solid bonding
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behavior of AZ31 Mg alloy during porthole die extrusion process. Int. J. Adv. Manuf. Technol. 93, 2791-2799.
Zhang, X., Cheng, Y., 2017b. Influence of inner fillet radius on effective strain
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homogeneity in equal channel angular pressing. Int. J. Adv. Manuf. Technol. 92, 4001-4008.
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Zhang, X., Yang, T., Castagne, S., Wang, J., 2011. Microstructure; bonding strength and
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thickness ratio of Al/Mg/Al alloy laminated composites prepared by hot rolling. Mater. Sci.
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Eng. A 528, 1954-1960.
Zhang, X., Yang, T., Liu, J., Luo, X., Wang, J., 2010. Mechanical properties of an
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Al/Mg/Al trilaminated composite fabricated by hot rolling. J. Mater. Sci. 45, 3457-3464.
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List of Figures.
Fig. 1. The experimental setup and material flow behavior during PCE process Fig. 2. SEM micrographs and EDS mappings across Al/Mg interface of (a) PCE340, (b) PCE370 and (c) PCE400.
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Fig. 3. Thickness of Al/Mg interface of (a) PCE340, (b) PCE370 and (c) PCE400. Fig. 4. EDS line scanning across Al/Mg interface of (a) PCE340, (b) PCE370, and (c) PCE400.
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Fig. 5. Inverse pole figure maps of the grain morphology in Al layer of (a) PCE340, (b) PCE370, and (c) PCE400.
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Fig. 6. Grain size distributions in Al layer of (a) PCE340, (b) PCE370 and (c) PCE400, and
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the (d) volume fraction of DRX.
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Fig. 7. The φ2=0°, 45° and 65° ODF sections of (a) PCE340, (b) PCE370 and (c) PCE400.
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Fig. 8. Schematic illustration of the main texture components in ideal FCC materials.
and (c) PCE400.
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Fig. 9. Relative frequency of misorientation angles in Al layer of (a) PCE340, (b) PCE370
Fig. 10. Inverse pole figure maps of the grain morphology in Mg layer of (a) PCE340, (b)
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PCE370, and (c) PCE400.
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Fig. 11. SEM micrographs of the precipitate distribution in Mg layer of (a) PCE340, (b) PCE370, (c) PCE400. Fig. 12. Pole figures in Mg layer of (a) PCE340, (b) PCE370 and (c) PCE400.
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Fig. 13. Hardness distributions across the Al/Mg bonding interface of the extruded laminates.
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List of Tables.
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Table 1 Chemical compositions (wt.%) of the as-received AZ91 Mg alloy and 6063 Al alloy.
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Table 2 The fractions of texture components in the Al layer.
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Fig. 1. The experimental setup and material flow behavior during PCE process
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Fig. 2. SEM micrographs and EDS mappings across Al/Mg interface of (a) PCE340, (b)
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PCE370 and (c) PCE400.
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Fig. 3. Thickness of Al/Mg interface of (a) PCE340, (b) PCE370 and (c) PCE400.
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Fig. 4. EDS line scanning across Al/Mg interface of (a) PCE340, (b) PCE370, and (c)
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PCE400.
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Fig. 5. Inverse pole figure maps of the grain morphology in Al layer of (a) PCE340, (b)
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PCE370, and (c) PCE400.
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Fig. 6. Grain size distributions in Al layer of (a) PCE340, (b) PCE370 and (c) PCE400, and
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the (d) volume fraction of DRX.
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Fig. 7. The φ2=0°, 45° and 65° ODF sections of (a) PCE340, (b) PCE370 and (c) PCE400.
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Fig. 9. Relative frequency of misorientation angles in Al layer of (a) PCE340, (b) PCE370 and (c) PCE400.
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PCE370, and (c) PCE400.
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Fig. 10. Inverse pole figure maps of the grain morphology in Mg layer of (a) PCE340, (b)
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PCE370, (c) PCE400.
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Fig. 11. SEM micrographs of the precipitate distribution in Mg layer of (a) PCE340, (b)
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Si
Fe
Cu
Mn
Zn
Al
Mg
AZ91Mg
0.031
-
-
0.33
0.64
9.31
Bal.
6063 Al
0.45
0.35
0.10
0.12
0.12
Bal.
0.80
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Table 2 The fractions of texture components in the Al layer.
Y
Copper
{111}<011>
{111}<112>
{112}<111>
{123}<634>
PCE340
40.10
17.60
6.94
7.24
PCE370
56.80
15.40
PCE400
59.60
8.45
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Specimen
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S
3.28
5.84
7.73
5.50
S0924-0136(18)30132-8 https://doi.org/10.1016/j.jmatprotec.2018.03.027 PROTEC 15697
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
29-1-2018 16-3-2018 28-3-2018
Please cite this article as: Chen L, Tang J, Zhao G, Zhang C, Chu X, Fabrication of Al/Mg/Al laminate by a porthole die co-extrusion process, Journal of Materials Processing Tech. (2010), https://doi.org/10.1016/j.jmatprotec.2018.03.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of Al/Mg/Al laminate by a porthole die co-extrusion process Liang Chena, Jianwei Tanga, Guoqun Zhaoa,*, Cunsheng Zhanga, Xingrong Chua a
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry
* Corresponding author: Guoqun Zhao, Professor @ Shandong University.
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Email: [email protected], Tel: +8653181696577, Fax: +8653188392811.
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of Education), Shandong University, Jinan, Shandong 250061, PR China.
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Abstract
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Graphical Abstract
A porthole die co-extrusion (PCE) process was proposed to fabricate the Al/Mg/Al
laminate. The results showed that the laminate was successfully extruded without voids and
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cracks on the Al/Mg interface. The transition layer was formed and its thickness was increased with the increase of temperature. Partial dynamic recrystallization (DRX) occurred in Al layer, and the texture of Al layer has strong E {111}<011> and Y {111}<112> sheartyped components and relative weak Copper {112}<111> and S {123}<634> rolling components. The near complete DRXed grain structure was observed in Mg layer, and the average grain size increases with increasing temperature. Mg layer has strong basal plane
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texture with (0001) planes parallel to extrusion direction and prismatic planes with random distribution. At lower extrusion temperature, both the Al and Mg matrixes exhibited higher hardness. Furthermore, the hardness of Al/Mg interface is lower than that of the Al and Mg matrix. Keywords: Porthole die co-extrusion; Al/Mg/Al laminate; Microstructure; Mechanical
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property.
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1. Introduction
The consumption of Mg alloy is increasing in the fields of automobile and aerospace due to the advantages of low density, high mechanical properties, excellent electromagnetic shielding ability and good recyclability (Meng et al., 2017). However, the practical application of Mg alloy is limited by the concern about its poor corrosion resistance. Similar to Mg alloy, Al alloy is also a light weight material with high strength and excellent corrosion
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resistance (Paulisch et al., 2016). According to the previous study of Li et al. (2016),
Al/Mg/Al laminate is regarded as an effective method to improve the corrosion resistance and
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the formability of Mg alloy. With the demand of light weight design, the Al/Mg/Al laminate has important applications in the fields of aerospace, automobile and high speed train by replacing the utilization of Al alloy.
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The researchers have devoted their efforts to finding effective method for joining
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dissimilar Al/Mg alloys. Liu et al. (2009) reported that Al/Mg alloys can be joined through
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diffusion bonding method, and the formation of intermetallic phases was related to holding
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time. Yan et al. (2010) fabricated a composite Al/Mg plate by means of explosive welding. Liang et al. (2017) applied friction stir welding (FSW) to join 1060 Al and AZ31 Mg alloys,
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and found that the constitutional liquation occurred at the interface of Al/Mg joints. Zhang et al. (2010) fabricated an Al/Mg/Al composite with a trilaminate structure by hot rolling, and
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investigated its mechanical properties at quasi-static rates of strain. Chang et al. (2009) reported that Al/Mg laminate can be fabricated by accumulative roll bonding, and studied the
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texture evolution of both Al and Mg layers. Similarly, Wu et al. (2010) studied the microstructure and mechanical properties of Al/Mg laminate fabricated by accumulative roll bonding.
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Recently, the researchers attempted to fabricate Al/Mg composite through hot extrusion
process. Both Mg and Al alloys are favorable for hot extrusion, and the Al/Mg composite can be directly extruded from as-cast Mg and Al billets. Thus, it is expected that hot extrusion can be used in mass-production of Al/Mg composite due to its high efficiency. Xin et al. (2015) developed an accumulative extrusion bonding (AEB) process to produce Al/Mg multilayer plates with excellent layer-layer bonding, and studied the grain structure and textures of both
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layers. Tokunaga et al. (2014) placed an Al plate on the top of Mg billet, and successfully obtained a bar with Mg core and uniform Al coating. Mahmoodkhani et al. (2016) studied the material flow and metallurgical reactions during co-extrusion of an Al/Mg clad. On the other hand, the researchers have proved that the hybrid billet combining Al sleeve and Mg core is effective in producing Al/Mg composite. Feng et al. (2016) extruded a well bonded Al/Mg rod using the as-cast billet including 7050 Al sleeve and AZ31 Mg core. Negendank et al.
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(2012) investigated the effects of die angle on the formation of diffusion layer during
extrusion using hybrid Al/Mg billet. Priel et al. (2016) designed the configuration of the
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hybrid billet, and the Al/Mg composite rod with no cracks, tears, and voids was obtained. The above mentioned studies proved that the Al/Mg composite with good bonding interface can be successfully hot extruded. However, it is noticed that the preparation of hybrid billet
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especially the Al sleeve is difficult and wasteful. Besides, the impurity and oxidation between
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Al and Mg billets might have adverse effects on the bonding quality of the Al/Mg interface.
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In the authors’ recent study (Fan et al., 2017), the porthole die extrusion was employed
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to join dissimilar 1060/6063 Al alloys, and a sound bonding interface was obtained. During such process, the surfaces of 1060 and 6063 Al billets were remained in the container, and
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only the fresh metal streams were solid bonded in the welding chamber under the condition of high temperature and pressure. Thus, the influence of oxidation on bonding quality was
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avoided. Moreover, the billets have simple cylindrical shape, which is easy for preparation. In order to further improve the boding quality, the structure of porthole die and process
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parameters such as ram velocity, extrusion ratio, and billet temperature should be well designed and controlled (Chen et al., 2015; Zhang et al., 2017a). In this study, a porthole die co-extrusion (PCE) process was proposed to fabricate the
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Al/Mg/Al laminate. The extrusion experiments were carried out at varied temperatures of 340 o
C, 370 oC and 400 oC, respectively. The morphology of Al/Mg interface and element
diffusion were analyzed. The grain structure and micro-texture in Al layer and Mg layer adjacent the interface were well examined. The micro-hardness across the bonding interface was measured. Based on the above works, the effects of billet temperature on microstructure and hardness of the extruded laminate were discussed. Importantly, it is proved that the
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Al/Mg/Al laminate with good bonding interface can be obtained through PCE method, and the microstructure of both Al and Mg layers was improved. 2. Experimental procedures
The 6063 Al and AZ91 Mg were used to fabricate Al/Mg/Al laminate, and the chemical compositions of both alloys are listed in Table 1. The as-cast 6063 ingot was homogenized at
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480 oC for 12 h and then cooled down to room temperature in the air. Similarly, the homogenization for as-cast AZ91 ingot was carried out at 420 oC for 10 h followed by air
cooling. Then, the cylindrical billets for 6063 and AZ91 alloys were machined for extrusion
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experiment.
Table 1 Chemical compositions (wt.%) of the as-received AZ91 Mg alloy and 6063 Al alloy. Si
Fe
Cu
Mn
Zn
Al
Mg
AZ91 Mg
0.031
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0.33
0.64
9.31
Bal.
6063 Al
0.45
0.35
0.10
0.12
0.12
Bal.
0.80
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The designed experimental setup and the material flow behavior during the proposed PCE process are shown in Fig. 1, where ED and TD indicate the extrusion direction and
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transverse direction, respectively. As is seen, the setup mainly consists of the components of ram, container, upper die and lower die. Three cavities with the inner diameter of 20 mm were
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designed in the container. Accordingly, the upper die has three portholes with the inner diameter of 14 mm and the welding chamber with the height of 10 mm. The bearing in lower
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die is used to control the final dimension of the extruded laminate. In this study, the plateshaped Al/Mg/Al laminate with the rectangular cross-section of 18×5 mm is extruded out
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from die bearing.
The PCE experiments were conducted using an electro-hydraulic servo pressing machine
of 200 tons. Firstly, two Al billets (ϕ20×62 mm) and one Mg billet (ϕ20×52 mm) were placed into the cavities of the container. The height of Mg billet is lower than that of the Al billet, in order to ensure that all metal streams can be extruded out simultaneously. Then, the billets, container, and dies were heated to the experimental temperature by a heating coil and held for
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15 min to have a uniform temperature distribution. After that, with the pressure from the stem, the fresh metal streams flowed into the portholes, and were solid bonded inside the welding chamber. Finally, the Al/Mg/Al laminate was extruded out from the die bearing. After extrusion experiment, the dies and extruded laminate were immediately quenched into
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iced water.
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Fig. 1. The experimental setup and material flow behavior during PCE process
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The process parameters such as ram velocity, extrusion ratio, billet temperature, and
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container temperature significantly affect the microstructure and the bonding quality of the extruded profile. In this study, the effect of billet temperature was studied, and the PCE experiments were performed at various temperatures of 340 oC, 370 oC and 400 oC.
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Accordingly, the extruded laminates were designated as PCE340, PCE370, and PCE400,
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respectively. The morphology and thickness of Al/Mg bonding interface were examined using scanning electron microscopy (SEM), and the element distribution across the Al/Mg interface was analyzed by means of energy dispersive spectroscopy (EDS). The grain structure and micro-texture of the Al and Mg layers near the Al/Mg interface were examined using electron
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back-scattered diffraction (EBSD). The Al and Mg specimens for EBSD analysis were firstly grounded and mechanically polished. Then, the Al specimens were electro-polished in the solution of 10 ml perchloric acid and 90 ml ethanol at 20 V for 5 s, and the Mg specimens were electro-polished using the mixture of 800 ml ethanol, 100 ml propanol, 18.5 ml distilled water, 10 g hydroxyquinoline and 75 g citric acid (ACII electrolyte) with the cooling of liquid nitrogen at 20 V for 40 s. The EBSD data was analyzed by Channel 5 software. Moreover, the 6
micro-hardness of the extruded laminate was measured at the interval of 0.5 mm along TD direction. 3 Results and discussion 3.1 Al/Mg bonding interface
The morphology of Al/Mg bonding interface under various experimental temperatures is
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shown in Fig. 2, where the green and red points represent the distribution of Al and Mg elements, respectively. As is seen, an obvious transition layer without any voids and cracks is formed in the interface of all laminates, which implies that a sound interfacial bonding was
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obtained by the proposed PCE process. Moreover, the transition layer contains two distinct sub-layers, and the sub-layer close to Al layer is thicker than that close to Mg layer. This
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phenomenon is similar with the experimental results reported by Wu et al. (2015).
Fig. 2. SEM micrographs and EDS mappings across Al/Mg interface of (a) PCE340, (b)
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PCE370 and (c) PCE400. Twelve positions along ED direction of the extruded laminate were selected to measure
the thickness of transition layer, and the results is shown in Fig. 3. It is obvious that the thickness of transition layer is increased with increasing experimental temperature. The average thicknesses of transition layers are calculated as 1.26 m, 2.26 m, 2.97 m for laminates PCE340, PCE370 and PCE400, respectively. Based on the element distribution 7
shown in Fig. 2, the inter-diffusion of Al and Mg atoms occurred during PCE process. The diffusion distance was increased with increasing temperature, which results in the variation of transition thickness with temperature. It is known that the diffusion in solid materials can be described by Fick’s second law, and the diffusion coefficient becomes higher with increasing temperature. On the other hand, the transport of atoms usually occurs though flaws and gaps in the solid materials. During PCE process, large amount of dislocations and vacancies were
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formed in the bonding interface due to the severe plastic deformation. These additional
vacancies have low migration energy, since high pressure was applied on the material. Hence,
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the vacancies tended to move towards the defects such as dislocations and grain boundaries, and the system energy can be reduced by such kind of vacancy eliminating (Sauvage et al., 2005; Ma et al., 2015). The equilibrium concentration of vacancies is enhanced at higher
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temperature, according to the following equation,
Ev kT
(1)
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Cv Ae
where Cv is the equilibrium concentration of vacancies, A is a coefficient determined by
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vibrational entropy, k is boltzmann constant, T is temperature, and Ev is vacancy formation
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energy. As a result, with the increase of temperature, more vacancies are induced, which can accelerate the atom diffusion across the Al/Mg interface. On the other hand, the diffusion velocity of Mg atoms into Al matrix is faster than that of Al atoms into Mg matrix as reported
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by Negendank et al. (2012), which results in the thickness of sub-layer close to Al matrix is
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larger than that close to Mg matrix. Fig. 4 plots the EDS line analysis across the Al/Mg interface. In the transition layer, Al concentration decreases from Al matrix to Mg matrix, while the tendency of Mg concentration is opposite. The concentrations of Al and Mg vary continuously across the Al/Mg interface. According to the previous study of Matsumoto et al.
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(2005), it implies that the materials were miscible at the present temperature and the intermetallic was not formed in the transition layer. The similar results were reported by Zhang et al. (2011) that the intermetallic was not formed in the interface of 7075 Al and AZ31B Mg alloys fabricated by hot rolling.
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Fig. 3. Thickness of Al/Mg interface of (a) PCE340, (b) PCE370 and (c) PCE400.
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Fig. 4. EDS line scanning across Al/Mg interface of (a) PCE340, (b) PCE370, and (c) PCE400.
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3.2 Microstructure in Al layer
The microstructure in Al layer adjacent Al/Mg interface was examined by means of
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EBSD. The inverse pole figure maps of Al layer are shown in Fig. 5, where the high angle boundaries (HABs) with misorientation angles >15° are indicated by thick-black lines and low angle boundaries (LABs) with misorientation angles between 2°-15° are indicated by
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thin-grey lines, respectively. Moreover, the fraction of HABs is defined by f. As is seen, Al layer of each laminate exhibits a bimodal grain structure consisting of coarse elongated grains and fine equiaxed grains, which implies the occurrence of partial dynamic recrystallization (DRX). Moreover, large amounts of LABs were formed inside the elongated grains, which is the evidence of continuous dynamic recrystallization (CDRX) (Jamaati and Toroghinejad, 2014). The evolution of grain structure in Al layer during PCE process is explained as below.
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Firstly, the coarse equiaxed grains of the billet were elongated along ED direction because of the compression stress, and those elongated grains were transformed to strip-shaped coarse grains. Then, a large amount of LABs was formed due to the occurrence of dynamic recovery. Finally, the dislocations produced by strain hardening accumulated in the LABs, which leads to the transformation from LABs to HABs and the occurrence of CDRX. As a result, the extruded microstructure of Al layer consists of strip-shaped coarse grains and some amount of
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fine equiaxed grains.
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Fig. 5. Inverse pole figure maps of the grain morphology in Al layer of (a) PCE340, (b) PCE370, and (c) PCE400.
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Fig. 6 shows the grain size distribution and the volume fraction of DRX in Al layer. As
is seen, the average grain size increases from 4.05 m to 6.72 m with increasing temperature. The smallest grain size in PCE340 laminate is firstly attributed to the finer
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DRXed grains and low mobility of grain growth at lower temperature. Moreover, the severe pressure and shearing action occurred during bonding stage, which induces large amount of dislocations to the grains near the bonding interface. The elongated grains can be broken by means of dislocation cutting and grain rotation (Cheng et al., 2014). In this study, the pressure and shearing action becomes much stronger at lower extrusion temperature, which is favorable for the formation of fine grains by breaking the elongated grains. Thus, due to the
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finer DRXed grains, low grain growth rate and breaking of elongated grains, PCE340 laminate has the smallest grain size. Fig. 6 (d) shows that the volume fraction of DRX rises with increasing temperature. It is known that high deformation temperature is beneficial for the DRX occurrence of Al alloys (Chen et al., 2017). During higher temperature deformation, more energy is provided for the transformation of microstructure, which can promote the growth of sub-grains and the nucleation of new grains. Besides, it will be easier for
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dislocation to slip and climb at higher temperature, and such dislocation migration can lead to
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the merging of sub-grains and the transformation of LABs to HABs (Liao et al., 2017).
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Fig. 6. Grain size distributions in Al layer of (a) PCE340, (b) PCE370 and (c) PCE400, and the (d) volume fraction of DRX. The micro-texture in Al layer is analyzed based on the orientation distribution function
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(ODF) sections of φ2=0°, 45° and 65°, as shown in Fig. 7. The locations of main ideal orientations in ODF section are given in Fig. 8. The strong E {111}<011> and Y {111}<112> components and relative weak Copper {112}<111> and S {123}<634> components can be observed in all extruded laminates. Besides, PCE340 and PCE400 have weak R-Cube component, while PCE370 has weak R-Goss component. The fraction of main texture components in Al layer is listed Table. 2. The sum of fraction of shear-typed components
11
such as E and Y is 57.7%, 72.2% and 68.05% for PCE340, PCE370 and PCE400, respectively. The sum of fraction of rolling texture such as Copper and S is 14.18%, 9.12% and 13.23% for PCE340, PCE370 and PCE400, respectively. Consequently, the main textures of Al layer are characterized as shear-typed components, and also small fraction of rolling texture. The dominance of deformation textures in Al layer also indicates the occurrence of CDRX, which agrees well with the above discussion. In this study, strong shearing stress is
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imposed on Al alloy due to the friction and die constraint before bonding stage, and strong shear-typed textures are formed by the rotation of grains. These shear-typed E and Y
SC R
components can transform into each other under combined effects of shearing stress, shear strain rate and temperature. However, during the bonding stage, some of the shear-typed textures transform to rolling textures due to the action of plane strain. Moreover, this
U
phenomenon can contribute to an increase in the fraction of HABs (Kim et al., 2005). For
N
PCE340, more shear-typed textures transform into rolling textures, which leads to a higher
A
fraction of HABs, as shown in Fig. 9. The maximum texture intensities of PCE340, PCE370 and PCE400 are 4.67, 8.01 and 7.76, respectively. This is because the transformation degree
M
from shear-typed to rolling texture. More shear-typed textures transform to rolling textures for
A
CC E
PT
ED
PCE340, which lead to the lowest value of texture intensity.
12
IP T SC R U N
CC E
PT
ED
M
A
Fig. 7. The φ2=0°, 45° and 65° ODF sections of (a) PCE340, (b) PCE370 and (c) PCE400.
Fig. 8. Schematic illustration of the main texture components in ideal FCC materials.
A
Table 2 The fractions of texture components in the Al layer.
Specimen
Texture component (%) E
Y
Copper
S
{111}<011>
{111}<112>
{112}<111>
{123}<634>
PCE340
40.10
17.60
6.94
7.24
PCE370
56.80
15.40
3.28
5.84
PCE400
59.60
8.45
7.73
5.50
13
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Fig. 9. Relative frequency of misorientation angles in Al layer of (a) PCE340, (b) PCE370
SC R
and (c) PCE400.
3.3 Microstructure in Mg layer
Fig. 10 shows the grain morphology in Mg layer near the Al/Mg interface. The Mg layer
U
of all laminates exhibits uniform and fine equiaxed grains with the size of several microns,
N
which can be attributed to occurrence of complete DRX. The average grain sizes of PCE340,
A
PCE370 and PCE400 are calculated as 2.59 m, 3.24 m and 3.66 m, respectively. Fig. 11
M
shows the distribution of second phases in Mg layer. It can be observed that both coarse and relative fine Mg17Al12 particles distribute along the grain boundaries. Moreover, the amount
ED
of fine Mg17Al12 particles in PCE340 is higher than those of PCE370 and PCE400. In case of AZ series Mg alloy, Al atoms dissolved into Mg matrix during the homogenization treatment
PT
resulting in the Al segregation, and the Mg17Al12 precipitations can be dynamically formed in the Al enriched region during the following hot extrusion process (Kim et al., 2017). With the
CC E
increase of temperature, the solid solubility of Al atoms in Mg matrix is enhanced, and the amount of Mg17Al12 precipitations is significantly reduced. The grain size of Mg alloys after extrusion is strongly dependent on the amount and distribution of second phase precipitations.
A
The Mg17Al12 precipitates can promote the DRX nucleation, which is well known as the particle stimulated nucleation (PSN) effects. Moreover, the Mg17Al12 precipitates strongly inhibit the growth of DRXed grains by reducing the interfacial energy of grain boundaries. On the other hand, the grain growth might also occur after DRX, while the driving force for grain growth is reduced at low temperature. Due to the above mentioned reasons, the grain size in Mg layer is increased with increasing temperature.
14
IP T SC R U N A
A
CC E
PT
ED
PCE370, and (c) PCE400.
M
Fig. 10. Inverse pole figure maps of the grain morphology in Mg layer of (a) PCE340, (b)
15
Fig. 11. SEM micrographs of the precipitate distribution in Mg layer of (a) PCE340, (b) PCE370, (c) PCE400. The (0001) and (10-10) pole figures of Mg layer are plotted in Fig. 12. Extrusion temperature has significant effects on the texture evolution of Mg alloys. As is seen, all laminates have a strong basal plane texture with (0001) planes parallel to ED direction, which
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is a typical texture for extruded Mg alloy as reported by Stanford and Barnett (2008). Besides, the prismatic planes with random distribution can also be observed. The basal pole has the
tendency to rotate away from TD direction. It is attributed to the activity of non-basal
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slip (Agnew et al., 2001), which is beneficial for the improvement of forming ability. The maximum texture intensity of PCE340, PCE370 and PCE400 are 4.94, 7.79 and 7.82,
respectively. Although complete DRX occurred in all laminates, the DRXed grains are finest
U
in PCE340. Borkar et al. (2013) reported that fine DRXed grains have relatively more random
N
orientations than medium to large sized grains. Thus, a weaker texture was formed in PCE340
A
extruded at low temperature. With increasing temperature, the number of fine grains with
A
CC E
PT
ED
M
random orientation decreases, and the maximum texture intensity is increased.
16
Fig. 12. Pole figures in Mg layer of (a) PCE340, (b) PCE370 and (c) PCE400.
3.4 Micro-hardness
Fig. 13 plots the micro-hardness distribution across the Al/Mg interface along TD direction. The values of hardness are almost stable in the zones far away from the Al/Mg
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interface. It is noted that PCE340 extruded at lower temperature exhibits higher hardness in
the matrix of both Al and Mg layers than that of PCE370 and PCE400. As discussed above,
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the average grain size in both Al and Mg layers increases with increasing temperature, which results in the reduction of hardness (Yu et al., 2017). Moreover, the DRX degree also
increases with increasing temperature in both Al and Mg layers. The occurrence of DRX can
U
reduce the density of dislocations, and the work hardening during hot extrusion can be well relieved with higher DRX degree, which also result in the reduction of hardness. It is also
N
observed that the hardness on Al/Mg interface is lower than that of the matrix zones.
A
Generally, the hardness of 6063 Al alloy is in the range of 40-90 HV, and the hardness of
M
AZ91 Mg alloy is in the range of 70-170 HV (Zhang et al., 2017b; Roy et al., 2015). It is mentioned that the intermetallics were not formed in the transition layer, and it is not
A
CC E
PT
ED
expected that the transition layer exhibits high hardness.
Fig. 13. Hardness distributions across the Al/Mg bonding interface of the extruded laminates.
17
4. Conclusions
In this study, a PCE method was proposed to fabricate the Al/Mg/Al laminate. The effects of temperature on the Al/Mg interface and microstructure of the extruded laminates were investigated. The main conclusions are drawn as follows. (1) The Al/Mg bonding interface without voids and cracks was obtained using the
formed and its thickness increased with the increase of extrusion temperature.
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proposed PCE process. Due to the diffusion of Al and Mg atoms, the transition layer was
SC R
(2) The microstructure of Al layer consists of both coarse elongated and fine equiaxed
grains, which indicates the occurrence of partial DRX. Fine grains with uniform distribution were observed in Mg layer due to the complete DRX. Moreover, the grain size of both Al and
U
Mg layers increased with the increasing temperature.
N
(3) The texture in Al layer consists of strong E {111}<011> and Y {111}<112> shear-
A
typed components and relative weak Copper {112}<111> and S {123}<634> rolling components. Mg layer has strong basal plane texture with (0001) planes parallel to ED
M
direction, and also some prismatic planes with random distribution
ED
(4) The micro-hardness in both Al and Mg matrixes is higher after low temperature extrusion due to the finer grain size. The hardness in the Al/Mg interface is lower than that in
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PT
the matrix, since the intermetallics were not formed.
Acknowledgements The authors would like to acknowledge the financial support from National Natural Science
Foundation of China (U1708251), China Postdoctoral Science Foundation (2017M622194) and The
A
Fundamental Research Funds of Shandong University (2017JC005).
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18
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J. Alloy. Compd. 555, 219-224. Chang, H., Zheng, M., Gan, W., Wu, K., Maawad, E., Brokmeier, H.G., 2009. Texture
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Fan, X., Chen, L., Chen, G., Zhao, G., Zhang, C., 2017. Joining of 1060/6063 aluminum
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Liang, Z., Qin, G., Ma, H., Yang, F., Ao, Z., 2017. The constitutional liquation at the
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Liao, B., Wu, X., Yan, C., Liu, Z., Ji Y., Cao, L., Huang, G., Liu, Q., 2017.
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Microstructure characterization of Al-cladded Al-Zn-Mg-Cu sheet in different hot
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deformation conditions. Trans. Nonferrous Met. Soc. China 27, 1689-1697. Liu, L., Zhao, L., Xu, R., 2009. Effect of interlayer composition on the microstructure
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Matsumoto, H., Watanabe, S., Hanada, S., 2005. Fabrication of pure Al/Mg-Li alloy clad
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Negendank, M., Mueller, S., Reimers, W., 2012. Coextrusion of Mg-Al macro composites. J. Mater. Process. Technol. 212, 1954-1962. Paulisch, M.C., Lentz, M., Wemme, H., Andrich, A., Driehorst, I., Reimers, W., 2016. The different dependencies of the mechanical properties and microstructures on hot extrusion and artificial aging processing in case of the alloys Al 7108 and Al 7175. J. Mater. Process.
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Zhang, X., Yang, T., Castagne, S., Wang, J., 2011. Microstructure; bonding strength and
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List of Figures.
Fig. 1. The experimental setup and material flow behavior during PCE process Fig. 2. SEM micrographs and EDS mappings across Al/Mg interface of (a) PCE340, (b) PCE370 and (c) PCE400.
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Fig. 3. Thickness of Al/Mg interface of (a) PCE340, (b) PCE370 and (c) PCE400. Fig. 4. EDS line scanning across Al/Mg interface of (a) PCE340, (b) PCE370, and (c) PCE400.
SC R
Fig. 5. Inverse pole figure maps of the grain morphology in Al layer of (a) PCE340, (b) PCE370, and (c) PCE400.
U
Fig. 6. Grain size distributions in Al layer of (a) PCE340, (b) PCE370 and (c) PCE400, and
N
the (d) volume fraction of DRX.
A
Fig. 7. The φ2=0°, 45° and 65° ODF sections of (a) PCE340, (b) PCE370 and (c) PCE400.
M
Fig. 8. Schematic illustration of the main texture components in ideal FCC materials.
and (c) PCE400.
ED
Fig. 9. Relative frequency of misorientation angles in Al layer of (a) PCE340, (b) PCE370
Fig. 10. Inverse pole figure maps of the grain morphology in Mg layer of (a) PCE340, (b)
PT
PCE370, and (c) PCE400.
CC E
Fig. 11. SEM micrographs of the precipitate distribution in Mg layer of (a) PCE340, (b) PCE370, (c) PCE400. Fig. 12. Pole figures in Mg layer of (a) PCE340, (b) PCE370 and (c) PCE400.
A
Fig. 13. Hardness distributions across the Al/Mg bonding interface of the extruded laminates.
23
A ED
PT
CC E
IP T
SC R
U
N
A
M
List of Tables.
24
Table 1 Chemical compositions (wt.%) of the as-received AZ91 Mg alloy and 6063 Al alloy.
A
CC E
PT
ED
M
A
N
U
SC R
IP T
Table 2 The fractions of texture components in the Al layer.
25
IP T SC R U N A M ED
A
CC E
PT
Fig. 1. The experimental setup and material flow behavior during PCE process
26
27
A ED
PT
CC E
IP T
SC R
U
N
A
M
Fig. 2. SEM micrographs and EDS mappings across Al/Mg interface of (a) PCE340, (b)
A
CC E
PT
ED
M
A
N
U
SC R
IP T
PCE370 and (c) PCE400.
28
IP T
A
CC E
PT
ED
M
A
N
U
SC R
Fig. 3. Thickness of Al/Mg interface of (a) PCE340, (b) PCE370 and (c) PCE400.
29
IP T SC R U N
A
Fig. 4. EDS line scanning across Al/Mg interface of (a) PCE340, (b) PCE370, and (c)
A
CC E
PT
ED
M
PCE400.
30
31
A ED
PT
CC E
IP T
SC R
U
N
A
M
Fig. 5. Inverse pole figure maps of the grain morphology in Al layer of (a) PCE340, (b)
A
CC E
PT
ED
M
A
N
U
SC R
IP T
PCE370, and (c) PCE400.
32
IP T SC R U
Fig. 6. Grain size distributions in Al layer of (a) PCE340, (b) PCE370 and (c) PCE400, and
A
CC E
PT
ED
M
A
N
the (d) volume fraction of DRX.
33
IP T SC R U N A M ED
A
CC E
PT
Fig. 7. The φ2=0°, 45° and 65° ODF sections of (a) PCE340, (b) PCE370 and (c) PCE400.
34
IP T SC R U N A M
A
CC E
PT
ED
Fig. 8. Schematic illustration of the main texture components in ideal FCC materials.
35
IP T SC R U N A M ED PT CC E A
Fig. 9. Relative frequency of misorientation angles in Al layer of (a) PCE340, (b) PCE370 and (c) PCE400.
36
37
A ED
PT
CC E
IP T
SC R
U
N
A
M
IP T SC R U N A
A
CC E
PT
ED
PCE370, and (c) PCE400.
M
Fig. 10. Inverse pole figure maps of the grain morphology in Mg layer of (a) PCE340, (b)
38
IP T SC R U N A M
A
CC E
PT
PCE370, (c) PCE400.
ED
Fig. 11. SEM micrographs of the precipitate distribution in Mg layer of (a) PCE340, (b)
39
IP T SC R U N A M ED PT CC E A Fig. 12. Pole figures in Mg layer of (a) PCE340, (b) PCE370 and (c) PCE400.
40
IP T SC R U N A M ED PT CC E A Fig. 13. Hardness distributions across the Al/Mg bonding interface of the extruded laminates.
41
IP T SC R U N A M ED PT CC E A Table 1 Chemical compositions (wt.%) of the as-received AZ91 Mg alloy and 6063 Al alloy. Alloy
Si
Fe
Cu
Mn
Zn
Al
Mg
AZ91Mg
0.031
-
-
0.33
0.64
9.31
Bal.
6063 Al
0.45
0.35
0.10
0.12
0.12
Bal.
0.80
42
43
A ED
PT
CC E
IP T
SC R
U
N
A
M
Table 2 The fractions of texture components in the Al layer.
Y
Copper
{111}<011>
{111}<112>
{112}<111>
{123}<634>
PCE340
40.10
17.60
6.94
7.24
PCE370
56.80
15.40
PCE400
59.60
8.45
A
CC E
PT
ED
M
A
N
U
E
SC R
Specimen
IP T
Texture component (%)
44
S
3.28
5.84
7.73
5.50